Concerns about EB-1C Green Card Portability

How does EB-1C portability work?

I also have a question about the EB1C portability of the priority date. Say, If I have I-140 approved under EB1C and have a particular priority date. For some reason, if I need to or want to change employers and I do not qualify for EB1C with other employers. My new employer can file I-140 under EB-2, or I can file EB-2 NIW myself. I have both options. The question is whether I can use the priority date of the earlier approved I-140 petition under EB1C to the new employer's EB-2 petition and file AOS.